The present invention relates to the measurement of acoustic signals in a patient and more particularly, to the determination of the location of acoustic sensors in an array on a patient.
Acoustic signals generated by turbulent blood flow may be utilized for the non-invasive detection of stenosis. To detect acoustic signals generated by turbulent blood flow, an array of sensors are placed on a patient""s chest. Typically, these sensors have been configured in a fixed array such that the spatial relationship between the sensors was known. However, because patients differ in physical characteristics, it may be advantageous to use discrete sensors which may be positioned on a patient in an optimal xe2x80x9cacoustic windowxe2x80x9d for monitoring blood flow.
One problem which may arise in a system where discrete sensors are utilized to detect acoustic signals relates to the determination of the spatial relationship of the sensors. In order to utilize advanced beam forming techniques on the received signals at the sensors, it is preferred that the spatial relationship of the sensors be accurately determined. One technique which may be utilized to determine the locations of the sensors involves an articulating arm which is calibrated and used to make physical measurements of the sensor locations by placing the tip of the articulating arm at each of the sensor locations. However, the use of an articulating arm may be slow and the contact between the arm and the patient or the sensor may move the sensor and unfortunately, thereby, provide an inaccurate reading. Furthermore, if the locations of the sensors are dynamically determined, by whatever technique, such a determination may still not provide a sufficiently accurate spatial relationship of the sensors as it is the point of contact between the sensors and the patient which are utilized in beam forming. These contact points are obscured once the sensors are placed on the patient and, therefore, may be difficult, if not impossible, to directly measure.
In light of the above discussion, a need exists for improvements in the determination of the spatial relationship between sensors placed on a patient.
In light of the above discussion, it is an object of the present invention to provide for the determination of the spatial relationship of the contact points of sensors in an array without disturbing the sensors.
A further object of the present invention is to provide for the determination of the spatial relationship of the contact point of sensors in an array without requiring a fixed relationship between the sensors.
Still another object of the present invention is to provide for the determination of the spatial relationship of the contact point of sensors in an array even when the contact points of the sensors are obscured.
These and other objects of the present invention are provided by methods, systems and computer program products which determine an obscured contact point based on a visible portion of an acoustic sensor of a medical device contacting a patient by acquiring a first image containing an upper surface of the acoustic sensor from a first viewpoint and a second image containing the upper surface of the acoustic sensor from a second viewpoint different from the first viewpoint. The location of the centroid of the acoustic sensor is determined in three dimensional space. The centroid may be determined in three dimensional space by determining the centroid of the acoustic sensor in each image and then determining the location in three dimensional space from the locations in the images. A plane of the visible portion of the acoustic sensor is also determined based on the position of the upper surface of the acoustic sensor in the first image and the corresponding position of the upper surface of the acoustic sensor in the second image. The contact point of the obscured portion of the acoustic sensor may then be determined by projecting a predetermined depth through the centroid of the acoustic sensor in a direction having a predefined relationship with the plane of the visible portion of the acoustic sensor.
Because the contact point can be determined based on the image data, the sensor need not come into physical contact with any measuring devices which may cause the position of the sensor to change during measurement. Accordingly, the present invention provides for the determination of the position of the contact point of a sensor without physically disturbing the sensor. The location of a sensor may be determined using images taken each time the sensor is applied to a patient. Also, by determining the centroid of the sensor in a plane corresponding to the upper surface of the sensor and then projecting normal to that plane to a depth of the sensor, the contact point may be determined even when the contact point of the sensor is obscured (i.e. when the sensor is conformally engaged to the patient""s skin).
In a further embodiment of the present invention, the plane is determined by locating corners of the visible portion of the upper surface of the acoustic sensor based on the position of the acoustic sensor in the first image and the corresponding position of the uppers surface of the acoustic sensor in the second image. Lines which pass through the corners of the visible portion of the acoustic sensor are determined so that a cross product of the lines will result in a vector normal to the plane of the upper surface of the sensor. The contact point may then be determined by projecting the predetermined depth through the centroid of the upper surface in the direction of the cross product.
In a still further embodiment, the corners are located through the use of calipers which are orthogonal to the edges of the sensor in the image. The locations of the corners of the visible portion of the acoustic sensor are then determined by moving the calipers until they first contact the sensor. The contact point is then utilized as a corner point.
In a preferred embodiment of the present invention, the visible portion of the acoustic sensor is a quadrangle. In such a case the selected corners for the lines for use in the cross product are alternating (opposing) corners of the quadrangle.
In a still further embodiment of the present invention, contact points of a plurality of acoustic sensors are determined. The relative spatial relationship between the plurality of acoustic sensors may then be determined based on the determination of the contact point of each of the plurality of acoustic sensors.
By determining each of the contact points of the sensors in a sensor array in three dimensional space, the relative positions of the contact points of the sensors may be determined from the respective location in space of the contact point of the each sensor. Because the contact point of each sensor may be determined individually, the determination of the spatial relationship of the contact point of each of the sensors in the sensor array does not require a fixed relationship between the sensors.
In a particular embodiment of the present invention, the first image is acquired by a first camera and the second image is acquired by a second camera. Preferably, these images are acquired simultaneously. These cameras may be charge coupled device cameras. In any event, in such a two camera embodiment, the first camera and the second camera may be calibrated so as to determine the relationship between the first viewpoint and the second viewpoint. Such calibration may be accomplished by acquiring at the first camera a first reference image containing a predefined configuration of landmarks and acquiring at the second camera a second reference image containing the predefined configuration of landmarks. The spatial relationship between an image plane of the first camera and an image plane of the second camera may then be determined based on the first and second reference images.
In a particular embodiment of the present invention, the calibration may involve computing camera parameters for each of the first and second cameras. Object-space coordinates for each camera are then estimated using corresponding points from at least two images of the same scene. Calibration of the cameras is indicated by coincidence of the object space coordinates for the corresponding points.
In still further embodiments of the present invention, the sensor has a visible surface which is a retro-reflective surface. Furthermore, an infrared illumination source may also be provided and configured so as to illuminate the sensor. The combination of the reflective surface, the infrared illumination and an infrared filter may substantially increase the contrast in the image so that the upper surface of the sensor appears brighter than the rest of the image. Thus, the detection of the sensor in the image may be more accurate or simplified. In still another embodiment of the present invention, the location of sensors in the first and second images is determined by locating sensor positions in the first image and locating sensor positions in the second image. A correspondence between the sensor positions in the first image and the sensor positions in the second image is then determined so that a sensor position in the first image is associated with the corresponding sensor position the second image, Such a correspondence may be determined by determining an epipolar line for a sensor position in the first image. It may then be determined which of the sensor positions in the second image the epipolar line intersects. A sensor position in the second image which the epipolar line intersects is then associated with the sensor position in the first image.
In a specific embodiment of the present invention, the association of positions in the first image with positions in the second image may be accomplished by determining if a plurality of sensor positions in the second image intersect the epipolar line. Sensor positions in the first image are then associated with sensor positions in the second image which intersect the epipolar line in an order in which the sensor positions in the first image intersect the epipolar line. Furthermore, whether a sensor position intersects an epipolar line may be determined by determining if the epipolar line intersects a bounding box around a sensor position. Signed distances between the epipolar line and opposite corners of the bounding box are then determined. The epipolar line intersects the sensor position if the signed distances between the epipolar line and the opposite corners of the bounding box are of opposite signs.